Organisms which transmit a disease organism to crops are called

Organisms that transmit disease-causing agents (pathogens) from an infected host to a healthy crop or animal are called vectors. A vector does not cause the disease itself; it acts as a carrier or vehicle that moves the pathogen from one organism to another.

Common examples of vectors in crop agriculture include aphids (which transmit many plant viruses), whiteflies (which spread geminiviruses), and leafhoppers (which carry phytoplasmas). In animal agriculture, examples include tsetse flies (transmitting trypanosomes) and ticks (transmitting babesiosis).

The other options describe different biological roles:

• Bacteria are microorganisms that can themselves be pathogens (disease-causing agents), not the organisms that carry them.
• A host is the organism that harbours or is infected by the pathogen - the crop plant itself is the host in this context.
• A parasite is an organism that lives on or in a host and derives nourishment at the host's expense. While some parasites can also act as vectors, the term specifically describing the role of transmitting disease is "vector."